Many industrial processes and systems are temperature critical and require constant temperature monitoring. As an example, in the petrochemical industry catalytic reactors comprise pressure vessels containing catalyst material, and are intended to operate at high temperatures and pressure and accommodate a large product through put. Not only is the catalyst expensive, but reactor shut down or damage affects the quantity of finished product. The financial statement bottom line is generally a function of the quantity of such finished product. Various events can cause local temperature excursions, which may severely affect the reactor efficiency. Thus, measurements are necessary. Thermocouples have long been a vehicle for taking such measurements within a vessel. Examples of such prior methods are depicted in U.S. Pat. Nos. 3,262,502; 3,637,438; and 3,901,080. Since the mentioned excursions, or flare-ups, may occur throughout the volume of the vessel, the larger the number of measurement locations, the greater the opportunity to deal with a problem before it becomes critical. Attempts have been made to increase the number of thermocouple devices available for such measurements, as illustrated in U.S. Pat. No. 4,376,227. However, even that improvement did not provide for the kind of multi-level, horizontal temperature profile required and provided by this invention.